diff --git a/app/src/main/java/org/schabi/newpipe/about/LicenseFragment.java b/app/src/main/java/org/schabi/newpipe/about/LicenseFragment.java
index 272e27240..4f6a69dcc 100644
--- a/app/src/main/java/org/schabi/newpipe/about/LicenseFragment.java
+++ b/app/src/main/java/org/schabi/newpipe/about/LicenseFragment.java
@@ -1,5 +1,6 @@
 package org.schabi.newpipe.about;
 
+import android.app.Activity;
 import android.content.Context;
 import android.content.Intent;
 import android.net.Uri;
@@ -39,7 +40,7 @@ public class LicenseFragment extends Fragment {
      * @param license the license to show
      */
     public static void showLicense(Context context, License license) {
-        new LicenseFragmentHelper().execute(context, license);
+        new LicenseFragmentHelper((Activity) context).execute(license);
     }
 
     @Override
diff --git a/app/src/main/java/org/schabi/newpipe/about/LicenseFragmentHelper.java b/app/src/main/java/org/schabi/newpipe/about/LicenseFragmentHelper.java
index 726e97ec2..e4436322a 100644
--- a/app/src/main/java/org/schabi/newpipe/about/LicenseFragmentHelper.java
+++ b/app/src/main/java/org/schabi/newpipe/about/LicenseFragmentHelper.java
@@ -1,8 +1,11 @@
 package org.schabi.newpipe.about;
 
+import android.app.Activity;
 import android.content.Context;
 import android.content.DialogInterface;
 import android.os.AsyncTask;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 import android.support.v7.app.AlertDialog;
 import android.webkit.WebView;
 import org.schabi.newpipe.R;
@@ -10,26 +13,46 @@ import org.schabi.newpipe.util.ThemeHelper;
 
 import java.io.BufferedReader;
 import java.io.InputStreamReader;
+import java.lang.ref.WeakReference;
 
 public class LicenseFragmentHelper extends AsyncTask<Object, Void, Integer> {
 
-    private Context context;
+    WeakReference<Activity> weakReference;
     private License license;
 
+    public LicenseFragmentHelper(@Nullable Activity activity) {
+        weakReference = new WeakReference<>(activity);
+    }
+
+    @Nullable
+    private Activity getActivity() {
+        Activity activity = weakReference.get();
+
+        if (activity != null && activity.isFinishing()) {
+            return null;
+        } else {
+            return activity;
+        }
+    }
+
     @Override
     protected Integer doInBackground(Object... objects) {
-        context = (Context) objects[0];
-        license = (License) objects[1];
+        license = (License) objects[0];
         return 1;
     }
 
     @Override
     protected void onPostExecute(Integer result){
-        String webViewData = getFormattedLicense(context, license);
-        AlertDialog.Builder alert = new AlertDialog.Builder(context);
+        Activity activity = getActivity();
+        if (activity == null) {
+            return;
+        }
+
+        String webViewData = getFormattedLicense(activity, license);
+        AlertDialog.Builder alert = new AlertDialog.Builder(activity);
         alert.setTitle(license.getName());
 
-        WebView wv = new WebView(context);
+        WebView wv = new WebView(activity);
         wv.loadData(webViewData, "text/html; charset=UTF-8", null);
 
         alert.setView(wv);
diff --git a/app/src/main/java/us/shandian/giga/ui/adapter/MissionAdapter.java b/app/src/main/java/us/shandian/giga/ui/adapter/MissionAdapter.java
index af25eaecb..12c81c127 100644
--- a/app/src/main/java/us/shandian/giga/ui/adapter/MissionAdapter.java
+++ b/app/src/main/java/us/shandian/giga/ui/adapter/MissionAdapter.java
@@ -1,11 +1,14 @@
 package us.shandian.giga.ui.adapter;
 
+import android.app.Activity;
 import android.app.ProgressDialog;
 import android.content.Context;
 import android.content.Intent;
 import android.net.Uri;
 import android.os.AsyncTask;
 import android.os.Build;
+import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 import android.support.v4.content.FileProvider;
 import android.support.v4.view.ViewCompat;
 import android.support.v7.widget.RecyclerView;
@@ -24,6 +27,7 @@ import android.widget.Toast;
 import org.schabi.newpipe.R;
 
 import java.io.File;
+import java.lang.ref.WeakReference;
 import java.util.HashMap;
 import java.util.Locale;
 import java.util.Map;
@@ -46,13 +50,13 @@ public class MissionAdapter extends RecyclerView.Adapter<MissionAdapter.ViewHold
         ALGORITHMS.put(R.id.sha1, "SHA1");
     }
 
-    private Context mContext;
+    private Activity mContext;
     private LayoutInflater mInflater;
     private DownloadManager mManager;
     private DownloadManagerService.DMBinder mBinder;
     private int mLayout;
 
-    public MissionAdapter(Context context, DownloadManagerService.DMBinder binder, DownloadManager manager, boolean isLinear) {
+    public MissionAdapter(Activity context, DownloadManagerService.DMBinder binder, DownloadManager manager, boolean isLinear) {
         mContext = context;
         mManager = manager;
         mBinder = binder;
@@ -247,7 +251,7 @@ public class MissionAdapter extends RecyclerView.Adapter<MissionAdapter.ViewHold
                     case R.id.md5:
                     case R.id.sha1:
                         DownloadMission mission = mManager.getMission(h.position);
-                        new ChecksumTask().execute(mission.location + "/" + mission.name, ALGORITHMS.get(id));
+                        new ChecksumTask(mContext).execute(mission.location + "/" + mission.name, ALGORITHMS.get(id));
                         return true;
                     default:
                         return false;
@@ -352,18 +356,26 @@ public class MissionAdapter extends RecyclerView.Adapter<MissionAdapter.ViewHold
 
     }
 
-    private class ChecksumTask extends AsyncTask<String, Void, String> {
+    private static class ChecksumTask extends AsyncTask<String, Void, String> {
         ProgressDialog prog;
+        WeakReference<Activity> weakReference;
+
+        ChecksumTask(@NonNull Activity activity) {
+            weakReference = new WeakReference<>(activity);
+        }
 
         @Override
         protected void onPreExecute() {
             super.onPreExecute();
 
-            // Create dialog
-            prog = new ProgressDialog(mContext);
-            prog.setCancelable(false);
-            prog.setMessage(mContext.getString(R.string.msg_wait));
-            prog.show();
+            Activity activity = getActivity();
+            if (activity != null) {
+                // Create dialog
+                prog = new ProgressDialog(activity);
+                prog.setCancelable(false);
+                prog.setMessage(activity.getString(R.string.msg_wait));
+                prog.show();
+            }
         }
 
         @Override
@@ -374,8 +386,24 @@ public class MissionAdapter extends RecyclerView.Adapter<MissionAdapter.ViewHold
         @Override
         protected void onPostExecute(String result) {
             super.onPostExecute(result);
-            prog.dismiss();
-            Utility.copyToClipboard(mContext, result);
+
+            if (prog != null) {
+                Utility.copyToClipboard(prog.getContext(), result);
+                if (getActivity() != null) {
+                    prog.dismiss();
+                }
+            }
+        }
+
+        @Nullable
+        private Activity getActivity() {
+            Activity activity = weakReference.get();
+
+            if (activity != null && activity.isFinishing()) {
+                return null;
+            } else {
+                return activity;
+            }
         }
     }
 }
diff --git a/app/src/main/java/us/shandian/giga/ui/fragment/MissionsFragment.java b/app/src/main/java/us/shandian/giga/ui/fragment/MissionsFragment.java
index a13c8d237..2ff83086f 100644
--- a/app/src/main/java/us/shandian/giga/ui/fragment/MissionsFragment.java
+++ b/app/src/main/java/us/shandian/giga/ui/fragment/MissionsFragment.java
@@ -129,7 +129,7 @@ public abstract class MissionsFragment extends Fragment {
     }
 
     private void updateList() {
-        mAdapter = new MissionAdapter(mActivity, mBinder, mManager, mLinear);
+        mAdapter = new MissionAdapter((Activity) mActivity, mBinder, mManager, mLinear);
 
         if (mLinear) {
             mList.setLayoutManager(mLinearManager);